Carbon Dioxide Hits Atmospheric Milestone

The daily average concentration of carbon dioxide in the atmosphere at a benchmark U.S. monitoring site has reached 400 parts per million for the first time, U.S. government scientists said Friday.

The site at Mauna Loa, Hawaii, has been monitoring carbon dioxide since 1958 and recorded the milestone on Thursday, said the National Oceanic and Atmospheric Administration. Carbon dioxide is the most significant of the greenhouse gases...